As you probably know, Mozilla has switched to a rapid release development cycle, planning to release Firefox 5, 6 and 7 by the end of the year and they've released Firefox Beta (currently Firefox 5), Aurora (Firefox 6) and Nightly (Firefox 7) channels from where builds are gradually promoted until they become the new stable Firefox. See this for more info.

But unfortunately, you can't install all of them side-by-side. Installing Firefox via the Beta channel or Aurora channel PPA will replace your current stable Firefox installation. However, that's not the case with Firefox Nightly channel PPA - that can be installed side-by-side with a stable Firefox installation.

Upgrade To Firefox Aurora channel in Ubuntu

Remember, adding the Firefox Aurora Channel PPA will replace your current stable Firefox! This version of Firefox is not stable so you may find bugs! Use it at your own risk.

The Firefox Aurora channel PPA is currently available for Ubuntu 11.10, 11.04 and 10.10. The Ubuntu 10.04 package failed to build but it will probably be fixed soon.

Add the Firefox Aurora channel PPA and install Firefox 6.0 alpha 2 (that's the version available in the PPA at the time I'm writing this post but obviously in the future you'll get newer versions) using the commands below: